This invention relates in general to position sensing devices for mechanisms which contain a movable member. More specifically, this invention relates to an apparatus and method for generating digital electrical signals which are representative of the angular position and rotational velocity of a rotatable shaft or similar movable mechanical component in an electromechanical servomechanism.
Generally speaking, a servomechanism is a control system which controls an output physical quantity in response to an input command function. Frequently, the servomechanism is embodied as an electromechanical control system, wherein a mechanical output component is moved in accordance with an electrical input signal. However, non-mechanical output quantities, such as temperature, pressure, voltage, etc., may be controlled in response to non-electrical input functions. Usually, the servomechanism includes a closed feedback loop, wherein a signal which is representative of the current status of the output physical quantity is compared with the current input command function. If there is a difference between the two, an error signal is generated which tends to urge the output physical quantity into compliance with the input command function.
In a typical electromechanical servomechanism, the output mechanical component may be a shaft which is rotated about an axis in accordance with a predetermined electrical input position signal. The servomechanism can function to rotate the shaft at a certain rotational velocity, to move the shaft to a certain rotational position, or to apply a certain torque or tension to a load in response to the electrical input signal. For example, most electric motors contain a rotor shaft which is driven to rotate about an axis relative to a stationary stator. Many conventional electric motors are controlled so as to rotate the rotor shaft at a certain rotational velocity relative to the stator in response to electrical input signal. Such conventional electric motors can be used to rotate the shaft at a certain rotational velocity, to move the shaft to a certain rotational position, or to apply a certain torque or tension to a load in response to the electrical input signal.
In such servomechanisms, it is often desirable or necessary to know what the precise status of the output physical quantity is at any given point in time. As mentioned above, the servomechanism usually includes a closed feedback loop, wherein a signal representative of the current status of the output physical quantity is compared with the current input command function. In the electric motor example mentioned above, it is often necessary or desirable to know what the precise rotational velocity or position of the rotor shaft is relative to the stator. The rotational velocity or position signal can be used in several manners to improve the operation of the electric motor. For instance, the rotational position signal can be used to increase the accuracy of the rotational velocity or angular position of the rotor shaft relative to the stator. Also, the rotational position signal can be used to control the timing of the energizations and deenergizations of the windings of the electric motor in order to increase efficiency and reduce undesirable torque ripple.
The rotational position signal of the rotor shaft in an electric motor is usually expressed as an electrical signal, either analog or digital in nature, which is representative of the angular position of the rotor shaft relative to a predetermined starting or reference position. Thus, the rotational position signal can be expressed as an analog or digital signal which is representative of a number between 0.degree. and 360.degree., wherein 0.degree. represents the predetermined starting position or reference position of the rotor shaft relative to the stator. A number of devices are known in the art for generating electrical signals which are representative of the angular position of the rotor shaft relative to a predetermined starting or reference position.
A resolver is one well known device for generating electrical signals which are representative of the relative angular position of a rotatable shaft. Generally speaking, a resolver is an electromechanical transducer which generates one or more output signals which vary in magnitude with the rotational position of the rotor shaft. Typically, the resolver generates two analog electrical output signals, one which is representative of the sine of the relative angle of the rotor shaft, and the other which is representative of the cosine of the relative angle of the rotor shaft. A number of different resolver structures are known in the art for performing this general function. The analog output signals may be used to control the operation of the servomechanism. Alternatively, it is known to convert the analog electrical output signals to respective digital electrical signals for subsequent use by a digital computing device, such as a microprocessor, in controlling the operation of the servomechanism.
While generally effective for many applications, known resolvers can generate errors in the analog electrical output signals which can reduce the accuracy of the resultant rotor shaft position signal. For example, errors may be introduced within the structure of the resolver itself which can cause the two electrical output signals to vary in amplitude, offset, and quadrature. Similar errors may also be introduced in such signals when they are processed by the other components of the servomechanism. It is known to use electrical devices to attempt to compensate for such errors in the output signals from the resolver. However, while known error compensation devices are effective for correcting known errors which are relatively constant over a period of time, they are not well suited for correcting errors which may vary over time, such as may occur as a result of changes in temperature and the like. Thus, it would be desirable to provide an apparatus and method which is effective for correcting time varying errors which are generated in known resolvers so as to improve the overall accuracy of the position measurement and to permit the use of simpler and lower cost resolver structures.